In general, the cassette duplicator machines comprise a master or source unit and one of various slaves or recorder units. The master unit is adapted to read at a high speed a master magnetic tape, and send a sample of the read signal to each one of the slaves for recording in the other magnetic tapes.
An example of one of these machines is the Audio/Tek cassette duplicating system, model 1200B having a master unit which may transport a magnetic tape 1/4" wide at a speed 32 times faster than the normal speed, and drive up to ten slave units. The master unit reads the signal registered in the portion of the tape which is instantly passing across the reader head, whilst the rest of the tape is deposited in a bin. The movement of the master tape is non-stop and unidirectional, so that tape is continually loaded out of the bin to feed the reader head before being loaded back again to the bin. Each slave unit accepts cassette magnetic tape arranged in bulk in open reel. Typically, each reel may contain 2,400 meters of tape which allows many cassettes to be recorded without interruption, for example, 32 cassettes 24 minutes long on each side.
Other more industrial machines operate similarly, for example MTI Corporation's "System III", uses master tapes either 1/2" or 1" wide and approximately 0.0015" thick. Generally, the normal speed of the master tape is 33/4 ips or 71/2 ips, i.e. respectively 2 or 4 times faster than the commercial cassette normal universal speed which is 17/8 ips. In the cassette duplicator machine, the master tape is transported at a high speed of 240 ips, whilst the slave speed is either 60 ips or 120 ips, according to the type, which is respectively equivalent to 32 or 64 times faster than its normal speed.
There is another type of cassette duplicator machines, not so well known as those mentioned previously, manufactured mainly by RECORTEC, in which the master tape, instead of being endless and deposited in a bin, is transported between two reels, first in one direction and then in the other. That is, a musical programme is recorded in a set of slave tapes, by passing the master tape from one reel to the other; then the set of slave tapes just recorded are replaced by a new set of virgin tapes, and the latter are recorded backwards by returning the master tape to the original reel. Consequently, the programmes are recorded successively, moving the master tape alternatively to and fro.
In all the cases, the duplicating operation is carried out by reading the master tape to reproduce the useful signal which is to be recorded again in the slave tapes. As is well known in the art, a bias signal is added to the useful signal before the latter is sent to the slave recording heads.
Furthermore, in those cases in which the commercial tapes are provided in bulk form for eventually providing tens of cassettes, a cue signal is sent at the end of each programme. Once the entire pancake has been recorded, it is transferred to a fractionating machine which reads the tape to detect the cue signals, in response to which the tape is cut between programmes and then placed in its individual cassette compartment.
It should be fairly evident that is is highly impracticable to carry out a quality control on the recording in each cassette, after the duplication has ended. However, various factors may contribute to cause a deficient recording in terms of fidelity in the commercial tape, for which reason some sort of efficient quality control is deemed necessary. The most usual of these factors worth mentioning are: misalignment of the recorder heads in relation to the commercial tape or twisting of the latter, wear and/or dirt of the recorder head, inadequate bias signal in relation to the composition and quality of the commercial tape, wow and flutter of the transport speed of the commercial tape, low quality composition or dirt of the commercial tape, etc.